The video compression algorithms using block-based encoding techniques such as MPEG or H.26x are compression algorithms with loss of information, often leading to a degradation of the visual quality of the sequence of encoded and subsequently decoded images with respect to the sequence of original images.
One of the visual artefacts which are most frequently observed with these encoding techniques is referred to as blocking artefact. This artefact is notably due to the quantization of blocks of coefficients resulting from the discrete cosine transform DCT, which quantization is rougher as the compression rate is higher. As a result, the degradation of the visual quality may range from an imperceptible level for a low compression rate to a level which is very annoying to the viewer for a high compression rate.
International patent application WO 01/20912 (PHF99579) describes a method allowing detection and localization of a grid corresponding to blocking artefacts within a decoded digital image. This method uses a gradient filtering of the pixels of a digital image and, for this reason, necessitates a complete decoding of the encoded data stream comprising the digital images, which renders its use costly in terms of computing resources, notably during transcoding.
European patent application EP 1 202 577 (PHF000106) describes a method of measuring blocking artefacts which is more precise than the method mentioned above. However, this method always necessitates a complete decoding of the encoded data stream comprising the digital images, as well as supplementary direct and inverse discrete cosine transforms, which renders its use also more costly in terms of computing resources than the above-mentioned method.